This invention relates to the electrodeposition of silver and silver alloys and more particularly to the improved electrodeposition of silver and silver alloys with up to 5 percent alloying metal, employing soluble or insoluble anodes.
In the conventional electrolytic silver plating baths, the electrolytes have almost always been limited to cyanide types because of the high stability constant of the complex K[Ag(CN).sub.2 ].
Conventional potassium silver cyanide plating solutions excel in current efficiency, brightness of the silver and silver alloy deposit, throwing power and speed of electrodeposition.
These features of the cyanide silver and silver alloy plating electrolytes are due to the presence of CN.sup.- ions in the solutions and very effective organic and/or inorganic brighteners.
Such CN.sup.- ions react to form poisonous HCN. For this reason constant care must be taken and efficient ventilization supplied. Another disadvantage is the presence of CN.sup.- ions near to or directly on the anode and especially on an insoluble anode where ammonia and potassium carbonate are formed.
Because of the accumulation of potassium carbonate in the electrolyte, very costly and complicated processes must be applied to remove an excess of such by-product.
Thus, because of its very high toxicity and, other reason, the art of silver and silver alloy electroplating tries to avoid the use of cyanide compounds.
It is an object of this invention to provide a non-cyanide silver plating bath which will readily deposit a uniform, bright, pure silver or silver alloy layer over a variety of conductive surfaces.
It is another object to achieve good adhesion of silver and silver alloy deposits to copper and copper alloys without the application of a silver strike prior to actual silver plating, thereby eliminating one or two processing steps.
At the present time most patents direct to non-cyanide silver plating baths are based on ammonia complexes of silver in combination with a variety of conductivity salts.
These include:
Some silver formulations employ amide and amine complexes. These include:
It is an object of this invention to produce mirror bright silver and silver alloy electroplates which are equal to or better than presently used cyanide containing silver plating baths.
It is also the object of the invention to produce mirror bright silver and silver alloy deposits over wide ranges of current densities.
It is further among the objects of the invention to provide a non-cyanide silver plating bath which is able to produce pure mirror bright silver deposit without any alloying metals.
These and other objects will become apparent from the following description of the present invention.
This invention is concerned with the electrodeposition of silver and silver alloy deposits, using a non-cyanide electrolyte formulation. In accordance with this invention, and the examples which follow, we provide an electrolyte where silver is present in complexes with organic compounds of the following:
For example:
In our invention, silver is bonded in a complex with succinimide and its derivatives or compounds resulting from those described herein. It is used as a reaction product of water soluble silver salt and the imide without separation of a pure compound.
The ratio of silver to the complexing agent is 1 mol of silver to two mols of complexing compounds, but may be different in accordance with complexing agent used.
The alkali metal silver complex is soluble in water if the pH is adjusted from 6.0 to 14. However, the pH value may vary slightly in accordance with use of complexing compound and alkali metal.
According to our invention, the new non-cyanide silver and silver alloy plating bath contains (1) succinimide, or its derivatives, or compounds of related common formulas described in accordance with this invention, (2) alkali metal or ammonium hydroxide, (3) soluble or insoluble silver salt, (4) optional conductivity salt or salts, (5) alloying metal salt, and (6) brighteners which can be employed alone or in conjunction one with another.
The non-cyanide silver and silver alloy plating bath works at temperatures between 20.degree. - 40.degree. C (68.degree. - 103.degree. F) and cathodic current density between 0.1-3A per square decimeter (1-30 amperes per square foot). The cathode area to anode area ratio should not be lower than 1:1, but extremely high ratios, of 1:10 and more, could be advantageous. Cathode current efficiency, regardless of anodes used, is 90 to 100%.
Anode current efficiency in the case of soluble silver anodes is 90 to 100%.
During the electrolysis of the non-cyanide silver plating bath, the silver complex is the source of silver ions and, later, liberated succinimide serves as a complexing agent to bond the silver dissolved from the soluble silver anode. In the case of insoluble anodes, liberated succinimide serves as a complexing agent for a water soluble or insoluble silver replenishing salt.
The ratio of silver ions to succinimide should be about 1.0: 2.0, but can be as high as 1: saturation point. The concentration of conductivity salts which form a soluble silver salt is not critical. The concentration of conductivity salts which form an insoluble silver compound is also not critical, up to the point where it interferes with the solubility of silver anodes during the electroplating process.
Consequently, in accordance with this invention, it has been found that mirror bright silver deposit can be achieved by incorporating alkylene, alkylol or alkanol amines into non-cyanide silver plating baths based on silver complexes described herein.
Besides the above mentioned amines, the same but in some cases even improved results can be achieved by using alkylene polyamines, which contain at least one secondary amino group and at least one primary amino group, or polyimine compounds having molecular weights in the range from about 100 to 60,000 with emphasis on the lower molecular weight range from about 100 to 2,000. The most active polyimines are polymers called polyethyleneimines which are formed by polymerization of ethyleneimines, substituted ethyleneimines, or derived from the addition of ethyleneimine to organic or inorganic molecules.
Accordingly, with the present invention, the non-cyanide mirror bright silver plating bath consists of a composition as follows:
______________________________________ (1) Silver as succinimide complex 5 to 100 grams per liter (2) Succinimide or its derivatives 10 grams per liter to saturation (3) Conductivity salt 0 to 300 grams per liter (4) Alkali metal/hydroxide 5 to 200 grams per liter or ammonium/hydroxide (5) Brightener 0.001 to 50 grams per liter (6) Alloying metal 0 to 5 mol percent Water to 1 liter pH 6.0 to 14 ______________________________________
According to this invention, the following compounds and their derivatives can be used as brighteners for non-cyanide silver plating bath:
______________________________________ polyethylene imines polypropylenes imines in the molecular polyhdroxyethylene-imines weight range defined poly (hydroxyethyl ethyleneimines) ethyl amines propylamine ethylenediamine propylenediamine diethylenediamine triethylenetetramine tetraethylenepentamine pentaethylenehexamine imino-bis-propylamine dimethyl amine propylamine diethylpropylenediamine ______________________________________
The above described amine or imino compounds can be used in a non-cyanide silver plating formulation based on siler succinimide complexes as a single compound or combined with each other.
Similarly, the following alloying metals are useful: copper, cadmium, gold, palladium and antimony.
A variety of conductivity salts can be used: NO.sub.2.sup. -, OH.sup.-, NO.sub.3.sup.-, F.sup.-, CO.sub.3.sup.--, PO.sub.4.sup.---, HPO.sub.4.sup.--, SO.sub.3.sup.--, SO.sub.4.sup.--, NH.sub.2 SO.sub.3.sup.-, mono-, di-, or tricarboxylic acids and their hydroxy or amine derivatives. The operating characteristics of the aqueous electroplating baths, such as the maximum current density, the cathode current efficiency, the width of the pH range, the brightness of silver or silver alloy deposit and the stability of the electrolyte are vastly improved over existing non-cyanide silver and silver alloy plating electrolytes. Furthermore, silver and silver alloy deposits from the above mentioned electrolytes are at least equal to or better than a silver, or silver alloy deposit obtained from conventional cyanide electrolyte, with excellent adhesion when applied over brass and copper without a preliminary silver strike.
The following specific examples illustrate the formulation of the baths: